diff --git a/spec/ameba/rules/method_names_spec.cr b/spec/ameba/rules/method_names_spec.cr
new file mode 100644
index 00000000..4a35d8fd
--- /dev/null
+++ b/spec/ameba/rules/method_names_spec.cr
@@ -0,0 +1,55 @@
+require "../../spec_helper"
+
+module Ameba
+  subject = Rules::MethodNames.new
+
+  private def it_reports_method_name(content, expected)
+    it "reports method name #{expected}" do
+      s = Source.new content
+      Rules::MethodNames.new.catch(s).should_not be_valid
+      s.errors.first.message.should contain expected
+    end
+  end
+
+  describe Rules::MethodNames do
+    it "passes if method names are underscore-cased" do
+      s = Source.new %(
+        class Person
+          def first_name
+          end
+
+          def date_of_birth
+          end
+
+          def homepage_url
+          end
+
+          def valid?
+          end
+
+          def name
+          end
+        end
+      )
+      subject.catch(s).should be_valid
+    end
+
+    it_reports_method_name %(def firstName; end), "first_name"
+    it_reports_method_name %(def date_of_Birth; end), "date_of_birth"
+    it_reports_method_name %(def homepageURL; end), "homepage_url"
+
+    it "reports rule, pos and message" do
+      s = Source.new %(
+        def bad_Name
+        end
+      )
+      subject.catch(s).should_not be_valid
+      error = s.errors.first
+      error.rule.should_not be_nil
+      error.pos.should eq 2
+      error.message.should eq(
+        "Method name should be underscore-cased: bad_name, not bad_Name"
+      )
+    end
+  end
+end
diff --git a/src/ameba/rules/method_names.cr b/src/ameba/rules/method_names.cr
new file mode 100644
index 00000000..8f6c53ed
--- /dev/null
+++ b/src/ameba/rules/method_names.cr
@@ -0,0 +1,45 @@
+module Ameba::Rules
+  # A rule that enforces method names to be in underscored case.
+  #
+  # For example, these are considered valid:
+  #
+  # ```
+  # class Person
+  #   def first_name
+  #   end
+  #
+  #   def date_of_birth
+  #   end
+  #
+  #   def homepage_url
+  #   end
+  # end
+  # ```
+  #
+  # And these are invalid method names:
+  #
+  # ```
+  # class Person
+  #   def firstName
+  #   end
+  #
+  #   def date_of_Birth
+  #   end
+  #
+  #   def homepageURL
+  #   end
+  # end
+  # ```
+  struct MethodNames < Rule
+    def test(source)
+      AST::DefVisitor.new self, source
+    end
+
+    def test(source, node : Crystal::Def)
+      return if (expected = node.name.underscore) == node.name
+
+      source.error self, node.location.try &.line_number,
+        "Method name should be underscore-cased: #{expected}, not #{node.name}"
+    end
+  end
+end